Royal Botanic Gardens
Kew Gardens possibly has the widest variety of plants collected from all around the world, certainly the largest collection of live botanical species in the UK. As well as the outside park there are also several greenhouses including a high tech green house containing a large number of habitats.
If you are interested in gardens and plants, Kew is definitely worth visiting.
You need to buy tickets, which is unusual for Parks in London. Prices vary according to the season.
Kew Gardens also contains Kew Palace which has recently re-opened.